FAQ
TL;DR: Most buried-cable leaks show as insulation resistance below 1 MΩ; “insulation resistance should be greater than 1 MΩ.” Use a megohmmeter to test each core to earth/PE and compare readings. Low‑megohm results need action. [Elektroda, Jacek Rutkowski, #17596982]
Why it matters: This FAQ helps homeowners and electricians quickly diagnose house‑to‑garage underground cable faults and choose safe, effective tests.
Quick Facts
- Pass/fail guide: insulation to earth should exceed 1 MΩ; lower implies insulation damage or leakage paths. [Elektroda, Jacek Rutkowski, #17596982]
- New‑cable expectation: typical core‑to‑core insulation above 20 MΩ per kilometer when new. [Elektroda, Jacek Rutkowski, #17603167]
- Reference ground: even a weak earth electrode is often <1 kΩ and works for IR testing. [Elektroda, Jacek Rutkowski, #17596982]
- Tester behavior: modern insulation testers auto‑discharge to about 24 V after the test. [Elektroda, Jacek Rutkowski, #19868163]
- Risk sign: 3–10 MΩ readings suggest moisture between conductors and a failure risk. [Elektroda, rtvserwisant, post #17602990]
What’s the easiest way to detect underground cable leakage between house and garage?
Isolate the circuit, then run an insulation resistance test between each conductor and earth/PE. Readings should exceed 1 MΩ. Lower values indicate leakage or insulation damage. Use the building’s earthing system or a ground rod as the reference. This simple test is the fastest way to confirm a fault. [Elektroda, Jacek Rutkowski, #17596982]
Can I check it with an ammeter if all garage loads are unplugged?
Yes, for a quick check on single‑phase. Disconnect all loads and insert an ammeter in series on the feed. Any current above zero supports a leakage suspicion. Use full electrical safety precautions. This method does not quantify insulation resistance or locate faults. [Elektroda, arturdip, post #17594867]
Can I just watch the utility meter for consumption to confirm leakage?
Yes. Disconnect everything on the garage circuit and observe the meter. Any consumption indicates a leakage path or an unintended load. This is a simple field check when you can isolate that run. Verify isolation before relying on the reading. [Elektroda, kloszi, post #17594912]
How do I test insulation to ground if the ground rod is far away?
Use the PE conductor as your earth reference. The distance to the electrode is fine if PE is intact. A broken PE will invalidate the reading. Confirm continuity of PE before testing. [Elektroda, Krzysztof Kamienski, #17596383]
What insulation resistance value is a clear red flag?
Below 1 MΩ. Leakage is rarely a perfect short, but healthy insulation reads higher. Measure between the grounding system and each conductor. Low insulation resistance indicates damage or contamination. Investigate before energizing the cable again. [Elektroda, Jacek Rutkowski, #17596982]
Is a reading between 3–10 MΩ acceptable for a buried garage feed?
No. Values around 3–10 MΩ point to moisture between conductors. That condition often precedes a major failure. Plan remediation, drying, or cable replacement before it escalates. [Elektroda, rtvserwisant, post #17602990]
What test voltage should I use on a low‑voltage house‑to‑garage cable?
Do not assume 250 V is correct. Test voltages follow the cable’s insulation rating and standards. The 250 V setting relates to cables with about 2.5 kV insulation class. Use your tester’s guidance and the applicable standard. [Elektroda, KonradGatek, post #17601931]
How do I perform a basic megohmmeter test on this cable?
Use a megohmmeter to test each core to the others and to earth. “Measure the insulation of each wire in relation to the others and to ‘earth’.” How‑To: 1. De‑energize and disconnect loads at both ends. 2. Test L1, L2, L3, N against each other and each core to PE. 3. Record values and compare; investigate any low reading. [Elektroda, KaW, post #17597379]
Will a distant ground rod skew my readings?
Unlikely. Even a weak earth electrode often measures under 1 kΩ. Use it as a solid reference for insulation tests across many meters. Distance is not the limiting factor; continuity is. [Elektroda, Jacek Rutkowski, #17596982]
Do modern insulation testers auto‑discharge the cable after testing?
Yes. Modern testers discharge the cable to a safe potential, about 24 V, before signaling completion. “Only the old inductors didn’t do this.” Treat old crank‑type testers with extra caution. [Elektroda, Jacek Rutkowski, #19868163]
Is a "mag meter" the same as a megohmmeter?
Yes. The term refers to a mega‑ohm meter, used for insulation tests. Wording varies, but the function is identical. Use any reliable insulation resistance tester. [Elektroda, Jacdiag, post #19868200]
Do measurement voltage and time matter when interpreting IR results?
Yes. Always note the test voltage and how long you held it. Interpretation depends on both parameters. If results look marginal, repeat with proper voltage and timing. Keep those details in your report. [Elektroda, elpapiotr, post #17603031]
What IR values are typical for longer power cables?
A practitioner’s rule of thumb: about 100 MΩ for up to 1 km of cable. Treat it as orientation, not a strict pass/fail. Compare trend lines and installation context. [Elektroda, KonradGatek, post #17601931]
What should I expect from new cables versus aging ones?
New cables can exceed 20 MΩ per kilometer core‑to‑core. Aging, moisture, and contamination reduce readings over time. Track periodic measurements to catch deterioration early. [Elektroda, Jacek Rutkowski, #17603167]
If my cable reads "OK", could there still be an intermittent fault?
Yes. Moisture can create low‑megohm readings that worsen over time. Intermittent paths may pass once and fail later. Re‑test after wet conditions and plan corrective work. [Elektroda, rtvserwisant, post #17602990]
What if the garage feed isn’t directly from the utility meter?
Then the meter‑watching method will not help. Use an insulation resistance meter instead. Test each conductor to earth and between conductors to confirm leakage. [Elektroda, CiekawyŚwiata, #17596328]
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